Citric acid, gluconic acid, ascorbic acid, tartaric acid, certain phosphonates, and other chelant materials have been used in the past to stabilize iron in solution and thus prevent its precipitation in forms such as ferric hydroxide and ferric oxide, wherein iron is in the second or third oxidation state. Citric acid and such prior art chelants effectively maintain iron in solution by forming complexes therewith which are soluble in water and thereby, remain dissolved in water. Although citric acid and cognate chelants are effective as solubilizing agents for iron in solution, they are not antiscalants and are ineffective against scale such as calcium carbonate, calcium phosphate, calcium sulfate, magnesium hydroxide, and the like.
Furthermore, performance of the prior art chelants is strongly dependent on solution pH. For example, iron (III) stabilization at 2 ppm of citric acid was 79% at pH of 7 but only 2% at pH of 8. In other words, an increase in solution pH from 7 to 8 decreased citric acid performance approximately forty times.
U.S. Pat. No. 4,552,665 discloses a process for stabilizing dissolved manganese ion and its reaction products in an aqueous solution by adding thereto 0.1 to 20 ppm of a copolymer of an unsaturated carboxylic acid or its salt and an unsaturated sulfonic acid or its salt. Acrylic acid is an example of the unsaturated carboxylic acid and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) is an example of the unsaturated sulfonic acid. The disclosed copolymers include 3-component copolymers or terpolymers.
The Amick U.S. Pat. No. 4,711,725 discloses stabilization of an aqueous system by inhibiting precipitation of calcium phosphate with a copolymer antiscalant, said copolymer comprising 42 to 84% of (meth)acrylic acid and salts thereof, 11 to 40% acrylamido alkane, sulfonic acid, and 5 to 30% or one or more monomers selected from vinyl esters, vinyl acetate, and substituted acrylamide. The aqueous medium can contain or can be devoid of iron contamination.
Other cases are being filed concurrently for Messrs. Amjad and Masler which relate to the use of same or similar polymers. One of the other cases is entitled "Scale Control with Copolymers Containing Acrylamidoalkane Sulfonic Acid," and the other case is entitled "Terpolymers for Dispersing Particulates In An Aqueous Medium".